Recently, there have been increasing demands for handling a burst signal (a signal which intermittently appears) on an optical transmission path in the fields of optical subscriber system and optical interconnection system.
When the burst signal is handled, the frequency of appearing "1" and "0" on the transmission path generally has a wide range of variation. Therefore, an optical receiver handling the burst signal is required to employ the DC coupling system. When the DC coupling system is employed in a circuit, however, such problem comes up that varying temperature and varying power-supply voltage make an operating point of the circuit susceptible to the variations, thereby rendering the circuit difficult to stably amplify.
Moreover, in an optical transmission system utilizing a baseband digital signal, binary information of "1" and "0" is transmitted. Therefore, the system utilizes the presence or absence of optical signals for a modulated optical signal. If this optical signal is converted into an electric signal by an optical-to-electrical conversion device, the electric signal takes a form of mono-polarity signal which occurs on either side (of "+" or "-") when a value of "1" appears with reference to a value of "0". In order to correctly distinguish between a value of "1" and a value of "0", the distinguishing value should be set exactly equal to a center value thereof when the electric signal is converted into a pulse signal. In a system in which a signal occurs in a burst manner, by converting a mono-polarity signal, in accordance with the presence and absence of signal, into a bi-polarity signal having an amplitude component in both directions of "+" and "-" with reference to an output level when there is no signal input. As a result, in a case where a noise is superimposing on the signal, values of "0" and "1" can be distinguished from each other with an almost-equal probability by referring to the aforementioned output level when there is no signal input.
Furthermore, when the converted bi-polarity signal is further amplified with reference to the aforementioned output level when there is no signal input, information on the changing points from "1" to "0" and from "0" to "1" are stored, whereby distortion of pulse width can be suppressed. Moreover, even if each burst signal is different in input optical power level, pulse can be precisely reproduced.
Hereinafter, a conventional optical receiver in which a mono-polarity signal is converted into a bi-polarity signal and the DC coupling system is employed is described.
A first conventional example of an optical receiver in which the DC coupling system is employed is described in IEEE ISSCC97 FP15.4. In this optical receiver, an output signal of a transimpedance-type preamplifier having a photodiode connected is inputted into a maximum value storing circuit and a minimum value storing circuit a center value of output of the respective circuits is generated in a resistance voltage dividing circuit, Sand the output signal of the aforementioned preamplifier is amplified by using a limiter amplifier with reference to the generated center value. Thereafter, a plurality of amplifying parts constituted by the maximum value storing circuit, the minimum value storing circuit, and the limiter amplifier are connected in series so that the pulse signal can be reproduced.
In the above-described first conventional example, however, the maximum value storing circuit and the minimum value storing circuit cannot be structurally the same in most cases, and thus each of the circuits has its own characteristics of temperature variation. As a result, a reference voltage to signal input of the limiter amplifier is influenced, it accordingly gets difficult to use the optical receiver in a wide range of temperature. Further, when each of the maximum value storing circuit and the minimum value storing circuit has an offset, these offsets cannot be cancelled. Therefore, the reference voltage to signal input of the limiter amplifier is also influenced, and consequently it gets difficult for the optical receiver to stably amplify.
Next, as a second conventional example of the optical receiver in which the DC coupling system is employed, the optical receiver disclosed in U.S. Pat. No. 5,430,766 is described. This optical receiver is constituted by a circuit whose main components are a preamplifier having a differential amplifier and another differential amplifier connected in the following stage.
In the aforementioned second conventional example, an output of peak detecting part on the positive side of differential output signal is fed back to determine a reference input level of the differential amplifier constituting the preamplifier. If temperature variation or power-supply voltage variation is observed, this output variation of the peak detecting part affects reference input in the preamplifier. Therefore, it becomes difficult to precisely reproduce a signal in a wide range of temperature. Further, if a noise is mixed in a signal line, the peak detecting part might respond to the noise and store an inaccurate peak value. Also in this case, it is difficult to precisely reproduce a signal. In this example, in order to avoid an influence to be caused by background light, feedback control is realized, which controls a current source to eliminate an obtained difference of peak values of differential outputs of the preamplifier. However,as two feedback loops are resultantly required, it is difficult to optimally set time constants of the respective feed back loops.
A third conventional example is the optical receiver disclosed in Japanese Patent Laying-Open No. 7-231307. This optical receiver amplifies, in a differential amplifier, an output from a preamplifier to which a photodiode is connected and a signal from a reference voltage source which outputs a value being almost equal to an output signal level of the preamplifier, and inputs a positive-phase output and a negative-phase output of the differential amplifier respectively into identically-structured maximum value storing circuits so as to detect and store each maximum value thereof. Thereafter, the optical receiver extracts a difference of these maximum values, multiplies the difference by 0.5, and adds the multiplied value to an output of center value generating part of differential output. This added value is treated as reference input of a comparator to be connected in the following stage. A pulse signal is thus reproduced by inputting the positive-phase output of the differential amplifier into signal input of the comparator.
In the aforementioned third conventional example, by using two identically-structured maximum value storing circuits and by extracting a difference thereof, offsets and characteristics of temperature variation in the maximum value storing circuits can be cancelled. According to the third conventional example, however, a center value cannot be precisely generated in some cases due to varying device (resistance devices) constants used in the center value generating part which generates the center value between the positive-phase output and the negative-phase output of the differential amplifier. In these cases, a reference signal of the comparator is deviated from a correct center value of differential output, whereby a pulse width of the reproduced pulse signal might be distorted. Moreover, if a noise is superimposed on a power-supply line or a ground line, a noise superimposing on an output and a noise component superimposing on a reference signal in the differential amplifier cannot be generally coincided with each other. Therefore, reproduction of a pulse signal may not be correctly accomplished by the comparator.
A fourth conventional example is the optical receiver disclosed in Japanese patent Laying-Open No. 9-289495 (U.S. application corresponding thereto Ser. No. 08/803,927). This optical receiver detects and stores each maximum value of positive-phase output and negative-phase output of a differential amplifier for amplifying signals, and generates a reference signal with respect to a positive-phase output signal of the differential amplifier for amplifying signals via an amplifier for reference signals having an amplification factor of 0.5. The differential amplifier for amplifying signals and the amplifier for reference signals are provided with identically-structured current sources and load resistors so that an output voltage of the amplifier for reference signals which is receiving no signal input and an output voltage of the differential amplifier for amplifying signals which is receiving no signal input can be set equal. In this system, by using identically-structured two maximum value storing circuits and by extracting a difference thereof, offsets and temperature variation in the maximum value storing circuits can be cancelled. Moreover, this system does not require the center value generating part required in the third conventional example, and thus a transmission path for signals is free from connection. Therefore, the signals can be amplified without causing deterioration of frequency characteristics.
According to the foregoing fourth conventional example, the differential amplifier for amplifying signals and the amplifier for reference signals are provided with the identically-structured current sources and load resistors so as to coincide the respective offset voltages when there is no signal input. However, an error of the offset voltages caused by variations in device constants cannot be cancelled. Therefore, if the error of the offset voltage is conspicuous, reproduction of pulse signal taken place in the comparator to be connected in the last stage might suffer a bad influence.